Recent technological developments in information exchange systems have witnessed proposals to replace keyboards or scanners as the fundamental message entry device. Generally these proposals entail some form of coordinate position input transducer upon which the user enters graphics data by means of a stylus, such as a light pen, shorting conductive rod stylus, etc. An array of signal pick-up elements located within or coupled integrally to the transducer produce output signals representative of specified or selected coordinate locations over which the input coupling element (e.g. stylus) travels as the user writes the graphics on the transducer. These signals are then processed (e.g. encoded, stored, etc.) and may be used to address a matrix-type display device through which the graphics generated by the user may be displayed for viewing. In addition to cathode-ray tube displays which typically respond to user-generated graphic input signals produced by a light beam stylus, there have been proposed schemes whereby the display device is of a relatively compact size and lies directly beneath the input transducer, so that the stylus appears to create a graphics display image at its coordinate points of contact with the device. For an exemplary illustration of these types of devices, attention may be directed to U.S. Pat. Nos. 4,177,354 to Matthews and 3,944,740 to Murase et al. The former patent describes a device whereby a light pen is used to cause local conduction of the input transducer through a photosensitive layer separating a conductive and a resistive plane. The coordinates of the light pen stylus are derived from the voltage level detected at the top conductive plane, and then processed for display via a display assembly such as a CRT unit, upon which the graphics entry transducer may be placed. This type of device has the disadvantage of being light sensitive, so that high ambient light or a light-emitting display could degrade its performance, the light pen scans only a single matrix transducer point at a time and the processing of the data for storage and display, as well as the display itself (CRT), requires a significant amount of hardware and high power requirements.
The scheme proposed in U.S. Pat. No. 3,944,740 is an attempt at a reduced-size or compact configuration of a combined graphics input/output device and, for this purpose, it employs a transparent input pad mounted over the top of a plasma panel display. The input pad is a matrix of conductive rows and columns that are arranged so that a stylus having a conductive tip can short circuit a row electrode and a column electrode at its point of contact, with current conducted through the row and column electrodes representing the stylus location. The plasma display has an inherent memory capability which is employed to provide a one-for-one storage and display function for each short circuit contact point of the stylus. Because of its reliance on the memory property of the plasma display, selective control of the display graphics and addressing the display from external memory are not possible, so as to substantially limit its adaptability to other data I/O interfaces such as external communications and memory.
In addition to the above-described proposals, there are various other coordinate display or hand-held or touch-responsive transducer arrangements, some for input data resolving alone, others for graphics input and display. However, these devices also suffer from a lack of versatility (they are capable of locating only one coordinate point at a time) and consume considerable power and involve complex hardware, thereby reducing their cost effectiveness and practical utility for the graphics message writer. For an overview of such proposals, attention may be directed to the U.S. Pat. Nos. 3,757,322 to Barkan; 4,030,091 to Ngo; 3,342,935 to Leifer et al.; 3,530,241 to Ellis; 3,699,439 to Turner; 4,055,726 to Turner et al.; 3,958,234 to Hoo; and 4,121,204 Welch et al.